Fluoropolymers have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability, low friction and anti-stick properties, etc.
Commonly known or commercially employed fluoropolymers include polytetrafluoroethene (PTFE), copolymers of tetrafluoroethene (TFE, also known as tetrafluoroethylene) and hexafluoropropene (HFP, also known as hexafluoropropylene), such copolymers are also referred to as PEP polymers, copolymers of tetrafluoroethene and perfluoroalkoxy copolymers (also known as PFA), copolymers of ethene and tetrafluoroethene (also known as ETFE polymers), copolymers of tetrafluoroethene, hexafluoropropene and vinylidene fluoride (VDF) (such copolymers also referred to as THV) and polyvinylidene fluoride polymers (PVDF) and others.
Fluoropolymers have been used to improve the thermal and chemical resistance of a substrate or to provide anti stick or low friction properties to a substrate, for example by coating or impregnating the substrate with the fluoropolymers. The fluoropolymers may be applied to the substrate by liquid coating techniques if they are provided as liquid formulations like aqueous dispersions.
Fluoropolymer dispersions can be conveniently produced by aqueous emulsion polymerization employing fluorinated monomers, one or more radical reaction initiators and a suitable surfactant. Fluorinated alkanoic acids of the type CF3—(CF2)n—COO− with n being an integer of 6 to 8, and in particular perfluorooctanoic acid (n=6) and its salts, have been the surfactant of choice in the aqueous emulsion polymerization of fluoromonomers for the last decades. Recently also other types of fluorinated surfactants, like fluorinated polyether acids, have been employed. Using fluorinated surfactants increases the solid content (polymer content) of the dispersions and leads to the generation of polymers of desired molecular weight and particle size in acceptable yields and reaction rates. Methods have been developed to retain and recycle the fluorinated surfactants from fluoropolymer dispersions leading to dispersions of low content of fluorinated surfactants. Currently the most commonly used technique to reduce the content of fluorinated surfactants is their removal by ion-exchange chromatography as described, for example, in European Patent EP 1 155 055 B1. However, fluoropolymer dispersions depleted from fluorinated surfactants may have decreased shear stability. Stabilizing surfactants, typically non-ionic surfactants have to be added to increase the stability of such dispersions. In international patent application No. WO2011/014715A1 non aromatic polyols, like alkyl polyglycosides, have been described as useful stabilizing surfactants.
Aqueous dispersions of TFE-homo polymers and TFE-copolymers with a high content of TFE are in particular susceptible to coagulation induced by shear forces. Such polymers are most commonly used in coating applications which require stable dispersions. Therefore, there is a continuous need to further increase the shear stability of coating compositions based on fluoropolymer dispersions containing no or only little amounts of fluorinated surfactants.